Because they’re mutually exclusive. Death and suffering is essential to evolution but the Bible teaches death and suffering in humans, plants, and animals is a symptom of man’s sin. Because of that sin we’re separated from God and require a perfect sacrifice which was provided by Christ. If death and suffering were just a natural process whereby things are made better, Christ wouldn’t have to die and our god would be a big jerk.
God created a world and called it good. He told us he did it in 6 days and that it was perfect. Man messed it up so God sent a second Adam to redeem the world. That redemption will be manifest when Christ returns again.
http://www.answersingenesis.org/docs2004/1025symposium.asp
That doesn’t prove that creation and evolution can’t co-exist, especially in consideration of God’s infiniteness.
Six days to God could be millions of years.